Frequency comparison system



Oct. 3, 1950 c. w. HANSELL FREQUENCY CWARISON SYSTEM Filed July l, 1942 2 Sheet's-Sheevt 1 AiToRNEY Oct. 3, 1950 c. w. HANSELL 2,524,702

FREQUENCY COMPARISON SYSTEM Filed July 1, 1942 z's'heets-sneet 2 INVENTOR ATTORNEY Patented Oct. 3, 1950 clarence W. Hansen, Rocky PoinaN. Y., 'assigner to Radio Corporation of'America, a corporation of Delaware Application July 1, 1942, serial'No. 449,255'

The present invention relates to an ecient method of and means for comparing two currents of dierent amplitudes and different frequencies.

My United States Patents Nos. 2,095,980 and 2,207,540 describe arrangements for comparing the current from a source of constant frequency with a current from a source of variable frequency and for utilizing the beat frequency to control or stabilize the source of variable frequency. The present invention provides an alternative system which can be used also for many dilferent purposes.

One of the objects of the present invention is to provide means for determining which of the two frequencies is higher than the other.

Another object of the invention is to provide an aircraft with means for determining by radio waves whether it is approaching or receding from an object. I

Broadly stated, the present invention is based upon the observation that when two currents of different amplitudes and different frequencies are combined in a single circuit, there is a 90 phase difference between the modulations of amplitude and of phase of the stronger current by the weaker current, and there is a relative phase reversal as the frequency of the weaker current passes through the frequency of the stronger current. Putting it in other Words, whenever` two currents beat together the resultant current is always both amplitude and phase modulated. The modulations of amplitude and phase are different by 90, i. e., they occur at different times, different by the time of one-quarter cycle of the beat. If the weaker current has a lower frequency than the stronger current, then the phase rotation of the two modulations is in one direction (depending upon definitions) and if the weaker current has a higher frequency than the stronger current, then the phase rotation of the two modulations is in aniopposite or reverse direction.

According to the invention, a radio receiver is designed to beat two received carrier currents together in order to furnish two output currents, one due to the amplitude modulations and the other due to the phase modulations of the carrier currents. The relative timing or phase rotation of the two output currents is an indication of whether or not the weaker of the two currents has a frequency above or below the frequency of the stronger current.

In one embodiment 'of the invention, the twophase output of the receiver is applied to a suitable indicating device in order to determine which 9 Claims. (Cl. 343-8) current has the higher frequency. By utilizing the Doppler effect and employing direct and reected waves. it is thus possible todetermine whether an aircraft, equipped with the apparatus of the invention, is approaching or receding from an object, such as another aircraft.

In another embodiment of the invention, the indicating instrument is provided with contacts for automatically correcting the frequency of a transmitter if it departs from a reference frequency by more than any arbitrary amount.

Other objects, features and their advantages will appear from a readingl of the following description which is accompanied by a drawing, wherein Figs. 1 2, 3, 4, 5 and 6 show six different embodiments of the invention. p

Throughout the gures of the drawing the same parts are represented by the same reference numerals.

Referring to Fig. 1 in more detail, there is shown in its most simplified form, a receiver of the invention equipped to determine which of two received frequencies is higher than the other. Let us assume that the two frequencies to be compared by the receiver are a frequency of a remote transmitter and a reference.frequency, both of which are radiated from more or less remote points. With this assumption, normally the transmitter current will be the stronger of the two input currents to the' receiver. These two input currents may be in the transmitter output frequency range, or may be at'a multiple or submultiple of this range, as dictated by convenience, in each case. The receiver of the invention compries an antenna i which feeds into a heterodyne detector and amplifier 2, the latter beating the received currents against the currents from a local heterodyne oscillator 3, in order to produce in the output circuit d intermediate frequency energy which is amplified `by intermediate frequency amplifier apparatus 5.

So far, we have described a conventional superheterodyne receiver, except for the detecting circuits to be described later. Itis preferred that this receiver have an automatic volume control arrangement ink order to assure a substantially constant intermediate frequency current level at the output of the intermediate frequency amplifler 5. This automatic volume control may be any one of numerous conventional circuits and its presence is indicated by the letters AVCin the apparatus 5. Unlike the conventional superheterodyne receiver, however, the receiverof` Fig. 1 is provided with both an amplitude modulation detector 6 and a phase modulation detector 1. both apparatus 6 and 1 including suitable amtwo input currents.

pliflers following the respective detectors. The amplitude modulation detector 6 and the phase modulation detector 1 both share the intermediate frequency power output in circuit 8, and both operate simultaneously to give outputs in leads 9 and I0 at the beat frequency corresponding to the difference in frequencies between the The amplitude modulation detector 6 and the phase modulation detector 1 may comprise any suitable well known apparatus for performing their intended functions. As an illustration: the phase modulation detector 1 may take any of the suitable forms illustrated and described in Crosby U. S. Patents Nos. 2,060,611, November 10, 1936; 2,065,565, December 29, 1936; and 2;l0l,703December 7, 1937. The demodulated output due to changes in amplitude, will appear in leads 9 inthe output of apparatus 6, while the demodulated output due to changes in phase, will appear in the leads I in the output of apparatus 1.

The output currents from the two detectors in leads 9 and I0 will be substantially 90 different in phase from each other, regardless of the output beat frequency, so that the two output currents provide a two-phase current. The relative timing, or polarity, or direction of phase rotation of the two currents in' 9 and I0 reverses as the frequency of the weaker current is moved through the frequency of the stronger current, or vice versa. The two-phase output currents in leads 9 and I0 (preferably amplied) are applied to the two windings of a phase rotation indicator II having an armature similar in principle to that of a two-phase induction motor. The direction of torque produced upon the armature or moving element of the indicator to which the needle I2 is linked is determined by the relative timing or polarity or direction of phase rotation of the two currents. Consequently, the indicating element here represented by needle I2 will be deflected in one direction or the other, depending upon whether or not the frequency of the weaker current is above or below the frequency of the stronger current. Observation of this indication may be had by observing the needle I2 as it moves over the scale I3. If desired, the indicator II may be so designed that the torque increases with increase of frequency over a large range. Therefore, the amount of rotation of the indicator from its center position against the restoring force of a spring may be a measure of the difference of frequency between the two input currents, while the direction of the rotation will show whether the transmitter frequency is above or below a desired reference frequency.

Fig. 1 thus provides an indicating instrument which enables the operating personnel to observe frequency deviations of a transmitter and to correct for such deviations.

At this time it should be understood that the two input currents which are received upon the antenna I may either be of continuous wave character or be constituted by trainsof pulses, in the latter case of which it will be necessary to have the pulse rate or pulse frequency higher than the highest required beat frequency.

Fig. 2 is a modification of Fig. 1, and shows how the invention can be employed to automatically control the frequency of a radio transmitter when it varies from a standard or reference frequency. The radio transmitter is shown diagrammatically at I4 and the standard frequency source (which may be a crystal controlled type) is shown at I 6. rents from the radio transmitter I4 and the standard frequency I5 are impressed both upon the amplitude modulation detector and amplifier 6 and upon the phase modulation detector and amplifier 1. The outputs from the detectors 6 and 1 appearing respectively in leads 9 and I0 are applied to the two windings of a phase rotation indicator or motor I I which has its needle I2 designed to engage a pair of contacts I1 and I9, depending upon the direction of movement of the needle I2, upon movement of the armature of the phase rotation motor I I. Contacts I1 and I6 are connected to opposite terminalsof the field coil of a small motor I9, whose armature is linked to a variable reactance, here shown as a condenser I6. This condenser may be one of the frequency determining elements of the radio transmitter I4, and is varied in such direction as to restore the frequency of the radio transmitter to the assigned frequency upon departure from the frequency of the standard. I thus provide an automatic frequency control system. Although the ra dio transmitter I4 and the standard I5 are shown directly connected to the detectors 6 and 1 by means of leads, these elements may be coupled to the detectors in any suitable way, as by means of antenna coupling, in which case I could use the antenna and the heterodyne detector and amplier scheme of Fig. 1. Also a heterodyne frequency changer and amplifiers may be interposed between the sources of current I4 and I5 and the detectors 6 and 1.

Fig. 3 shows the invention applied to a frequency meter scheme. The system of this figure is designed to indicate whether the frequency of the radio transmitter I4 is higher or lower than or identical with the frequency of a standard source I5, and, if there is any diierence between the frequencies of these two sources, the extent of the deviation. As in Fig. 2, the outputs of the transmitter I4 and the source of standard frequency I5 are both applied to the amplitude modulation detector 6 and the phase modulation detector 1. The frequencies applied to the detectors 6 and 1 are preferably of an intermediate frequency character, for which reason it is preferable that there be a heterodyne oscillator and heterodyne detector and amplifier as Well as an intermediate frequency amplifier between sources I4 and I5 of the detectors 6 and 1. The apparatus for producing intermediate frequencies are well known and are not shown in this gure, although similar apparatus is shown in Fig. l. The outputs from the two detectors 6 and 1,

which appear in leads 9 and I0, respectively, are

substantially different in phase regardless of the output frequency, so that the two output currents in leads 9 and II] provide a two-phase current, in the same manner described above in connection with Figs. 1 and 2. This two-phase current is applied to the two-phase motor type of relay I I', substantially of the same type shown in Fig. 2. Relay II' is provided with contacts I1 and I8 which are alternately adapted to make contact with the needle I2 depending upon the direction of motion of the motor relay, the needle, in turn, being connected to the armature of the motor II. A reversible frequency meter 20 having a needle 2I movable over a scale 22 serves to indicate whether or not the frequency of the radio transmitter is above or below'the frequency of the standard I5 and the extent of deviation. A suitable zero point on the scale 22 will vrepresent the standard frequency. The frequency meter The output cur- 2,524,762 L a u two-phase output of the detectors 6 and 1, andl therefore depending upon the frequency relation between the weaker and stronger input currents to the apparatus constituting the amplitude and phase modulation detectors. deflection, just as in the conventional types ,of

frequency meter, is then a measure of thexbeat frequency.

In the arrangement of Fig. 3 it is assumed that currents from the transmitter I4 and the source I5 delivered to detectors 6 and 1, have constant amplitudes, which will usually be easyto accomplish in practice. As an aid to holding-the currents constant, if the transmitter powerv is subject to variations 0r changes in value, an amplitude limiter, or an automatic volume controlled amplifier may be inserted between transmitter I4 and the detectors 6 and 1.

Fig. 6 illustrates diagrammatically the applir cation of the invention to an airplane, for ,determining whether the airplane istraveling toward or away from an object. Such a scheme may serve to prevent collisions between planes or between an airplane and any other objectin its path of travel. The airplane is equipped both with a transmitter 3| for radiating over a directive antenna, 32 suitable waves in the direction of travel of the plane. The airplane is also provided with a, receiver 40 which is coupled to the receive ing antenna I for receiving the waves which are reflected from the object in the path of travel. Apparatus for the receiver 40 is shown in Figs. 4

and 5, but before entering -into a detailed de.

scription of this apparatus a general vexposition of the operation of the scheme shown in Fig. ol

will be given.

The waves radiated from the transmitting antenna 32 and impinging upon an object lEll'iu the path of travel of the plane, will be reflected from the object and will be collected by receiv-v ing antenna I' in the manner shown by the dash lines 42 of Fig. 6. The arrows on these dash lines serve to indicate the direction of travel of the waves between thejtransmitting antenna 32 and the receiving antenna I'. These antennas have been shown, by way of -example only, as

The amount of thetrolled by ax direct coupling provided within the plane itself, between the transmitter and receiver, which' is made tov oppose, or add to the unavoidable coupling normally existing between nearby antennas, even those which'are highly directive, This latter arrangement is preferred. y l

The receiver 40 is preferably of the superheterodyne type and is equipped both with an amplitude and a phase modulation detector to share the intermediate frequency output, in order to provide outputs `at the frequency corresponding tothe difference in frequency between the twov input vcurrents representative of the direct and reflected waves. It will be apparent that the direct'wave will kbethe stronger of the twofwaves,l while'the reflected wave will have a, frequency Y depending upon the motions lof the planes 30 and dipoles backed up by suitable reflectors to assure a unidirectional pattern. It should be understood that the representation of these antennas is merely diagrammatic and'that they may take other forms and be embodied in the wings of thefplane. Furthermore, it may be possible to employ a, single antenna structure for both transmission and reception. In many cases the antennas may be substantially non-directive when it is desired to observethe presence and motions of objects in a wide variety of directions. The antennas also provide a direct radiation, represented yby dash line 43 between the transmitting antenna 32 and the receiving antenna I'. Usually, the direct radiation may be obtained in sufllcient amount, as compared with the reflected radiation, without any special provision being made for it.

This direct radiation can, if desired, be con- 50. According to the Doppler principle, if the plane 30 is approaching the yobject 50, the fre-l quency ofthe reflected waves `will be higher than that ofr the direct waves; an'd if the .plane 30 is receding from the object 50, the reflected wave will have a frequency lower'than the'frequency of the direct wave. Hence, the two wave components, direct and `reflected (due to the Doppler effect) have a difference 'in frequency depending upon themotions of the planes.v

Fig. 4 illustrates one form of apparatus which may be used in the system of Fig. 6. In the apparatus of Fig. 4 a radio transmitter 3| supplies power-continuously to antenna 32 from which" :the power may be radiated predominantly in a forward direction. A portionof the radiated powerv will be reilected back to the directional receiving antenna I from reflecting objects, such as another airplane, while another larger portion will reach the receiving antenna by local coupling. The direct and reflected waves lreaching the receiving antennal I'. are heterodyned down .to an intermediate frequency and amplified by suitable apparatus 2, and applied to amplitude and phase modulation detectors 6 and 1, respectively, which provide a' two-phase frequency output as a.

result of the beating together of direct and reflected .wavesfrom antenna 32 to antenna I.

Assume, as an example, that the frequency vol' the waves transmitted from the antenna 32 is 3,000 megacycles, corresponding to a wave length of 0.1 meter. AIf the combined relative velocities of the two airplanes, toward or away from one another, is as much as 300 meters per second, corresponding to a relative combined velocity of about 670 miles per hour, or 335 miles perhour.

per plane, the Vmaximum beat frequency between directand reflected waves would be about 3,000y

cycles per second.

If amplified beat frequency energy from '01net of the detectors is applied to a -pair of earphones, worn by the pilot, heV will be able to hear the beat frequency `which results from the presen-ce of an object, such as another plane, which lies on or near his line of flight. v

can change the beat frequency. VIf he manoeuvers to Vincrease the frequency and the intensity of the beat frequency energy, he will be bringing the vtwo airplanes closer together whereas if he irranoeuvers to increase/the frequency but decrease the intensity, he will be bringing` them further apart. By using directive transmitting and receiving antennas the change in beat note l By turning his plane, to change the relative velocity, the pilotI 7 system. without any otherv means of detecting the presence of the other plane. Therefore the system as so far described enables a pilot, nying in 4dari: or fog, to nd. or to avoid, another plane. He may,` of course, be provided with a manually operated volume control to control the overall sensitivity of the system so that `planes or other objects may be detected at maximum distance but, at lesser distances, overloading of the receiver, or of the earphones may be avoided. Setting of the volume control, and lcudness of the beat note in the phones, then yprovide the pilot with a rough measure lof distance.

As a refinement, valuable under some circumstances, such as when the transmitter and receiver are at ground stations, or on` mobile objects which are hard to manoeuver, I may provide the operator with an additional means of determining whether or not the reflecting object is moving toward or away from the antennas of the system; One such additional means is illustrated in Fig. 4.

In the arrangement of Fig. 4 output from the amplitude and phase modulation detectors provides a two-phase audio output in which' the direction of phase rotation reverses as the frequency of the reflected waves moves through the frequency of the radiated waves. fore the direction of phase `rotation provides a means of indicating whether or not the refie'ct- I In the arrangement of Fig. 4, Vby way of illustration only, I have shown two commutators which make and breakcircuits through them at constant but different rates. The input to the earphones passes through onevcommutator or the other depending upon which vof contacts I1 and I8 is contacted by the moving.element of the phase rotation-relay Il'. The motion of the phase rotation relay, in turn, is of course determined by the direction of phase rotation of the amplified, two-phase, beat frequency currents supplied to it from the amplitude and phase modulation detectors 6 and 1.

The arrangement of Fig. 4 is a good one to use when`the productof wave frequency and maximum relative velocities is high so that the beat note frequency delivered to the earphones ranges up to fairly high values. When the prod- Y uct of wave frequencies and lrelative velocities is low, thereby resulting in low frequency beats between direct and reiected waves, thev arrangement of Fig. 5 is more suitable.

Referring to Fig. 5, assume as an example that the reflecting object is van airplane which has a certain natural or resonant frequency for the wings. The wings, depending upon their dimenfsions will be resonant as a half wave oscillator at some particular relatively low frequency. At this frequency the wings will provide approximately a maximum of reflection and a minimum of directivity of the reflected waves. `Since each type and size of plane has its own resonant frequency the operator, by switching from one transmitted frequency to another, may concentrate on observation to determine the presence Thereand relative motion of one type and sin of plane at a time.

other, up to say 300 `meters per second. corre-` sponding to a relativecombined speed of the two planes of about 670 miles per hour or 335 miles per hour for each plane, the maximum beat frequency would be, say. 5 cycles per second, which is .too lowV to be directly audible. In order to make the beats audible, I propose utilizing them to modulate an audible frequency in the range, let us say, for example, 400 to 1200 cycles per second. In the operation of the system ofFig. 5, the beat frequency current in the leads I0, in the output phase modulation detector 1, produced from lthe intermediate frequencies obtained from collecting on antenna l' the direct and reflected waves, is fed back to modulate an audio oscillator Il which, inA turn, modulates the phase of the local heterodyne oscillator 3'. The five cycle beat note. it will thus be evident, serves twopurposes; namely, to modulate the audio oscillator 5I and also to operate the phase rotation relay Il'. This relay Il by means of its contacts i1 and i8 determines the mean frequency of the audio oscillator 5|. The five cycle beat frequency preferably amplitude modulates the output from the audio frequency oscillator 5I tothe heterodyne oscillator $5 by substantially 100% and the modulated oscillator output fromaudiobscillator 8| then'lphase or amplitude modulates the heterodyne oscillator 3'. In the particular arrangement shown in Fig. 4, where the headphones Il for the airplane pilot are connected to the output of the phase modulation detector 1, it is assumed that the heterodyne oscillator I' is phase modulated. If the headphones 80 on the other hand are connected to the output of the amplitudel modulation detector 6,l then the heterodyne oscillator 3' should be amplitude modulated. v i

'I'he audio oscillator 5I is one which is not energized until one contact I1 or the other Il of the phase rotation relay Il' closes and then the audio oscillator il hask a diierent frequency, depending upon which contact closes. These contacts on the phase rotation relay `serve to change the constants of the frequency determining circuit for the audio oscillator 5 I, as well as to energize the audio oscillator. moving apart, the pilot may hear, say, 400 cycles current modulated at the beat frequency between the direct and reflected currents, whereas if they are coming together, he may hear, say. 1200 cycles current modulated at the beat frequency rate. The headphones are. of course, not f responsive to the five cycle beatl but are responsive only to the audio modulation of 400 or 1200 cycles. With this indication of direction and magnitude of relative motion of the piane, the pilot may readily manoeuver to increase or decreasey the distance between the planes.

If he changes direction, up or down, right or left, he can hear the change in beat frequency which tells him immediately whether he has turned more toward or more away from the other plane. In practice, it may beidesirable to use the wings of the plane both for transmitting and receiving purposes, and, if desired, balancing ar- If the planes are` detector for beating with the received waves to produce an intermediate frequency, an intermediate frequency amplifier, a phase detector and an amplitude modulation detector, individual couplings from the output `of said intermediate frequency amplifier to the inputs of said detectors, and a multi-winding phase rotation responsive device having different windings coupled to the different outputs of said detectors.

2. A frequency comparison system comprising means for beating together the waves to be compared, an amplitude modulation detector and a phase modulation detector, circuits coupling the inputs of both of said detectors to the output of said means, and a two-phase rotation device having one winding coupled to the output of one detector and another winding coupled to the output of the other detector.

3. A receiver having an antenna for receiving waves of more than one frequency, a heterodyne detector coupled to said antenna, a heterodyne oscillator coupled to said detector for beating with the received waves to produce an intermediate frequency, an intermediate frequency amplifier coupled to the output of said heterodyne detector, a phase modulation detector and an amplitude modulation detector, couplings from the output of said intermediate frequency amplifier to the input electrodes of said last two detectors, and a phase rotation responsive device coupled to the outputs of said last two detectors, a contact arm mechanically linked to the armature of said phase rotation responsive device, a pair of contacts oppositely disposed relative .to

said arm and adapted'to alternately engage said arm depending upon the direction of torque produced upon the armature of said device, an audio frequency oscillator coupled to said contacts and arranged to be controlled therefrom, said audio oscillator generating one frequency when said arm engages one of said contacts and another frequency when said arm engages the other of said contacts, a circut for modulating said heterodyne oscillator from said audio oscillator, and an audio frequency translating device coupled to the output of one of said detectors.

4. A receiver having an antenna for receiving waves of more than one frequency, a heterodyne detector coupled to said antenna, a heterodyne oscillator coupled to said detector for beating with the received waves to produce an intermediate frequency, an intermediate frequency amplifier coupled to theA output of said heterodyne detector, a phase modulation detector and an amplitude modulation detector, couplings from the output of said intermediate frequency amplier to the input electrodes of said last two detectors, and a phase rotation responsivedevice -coupled to the outputs of said last two detectors, a contact arm mechanically linked to the armature of said phase rotation responsive device, a pair of contacts oppositely disposed relative to said arm and adapted to alternately engage said arm depending upon the direction 0f torque produced upon the `armature of said device, lanaudio4 frequency oscillator coupled to said contacts and arranged to be controlled therefrom, said audio oscillator generating` one frequency when said arm engages one ofl said contacts and another frequencywhen said arm engages the other of said contacts, a circuit for lmodulating the phase of theheterodyneoscillator from said audio oscillator, and headphones V coupled to the output of said phase detector. y

d5. A receiver having an antenna'for receiving lwaves of more than one frequency, a heterodyne detector coupled to said antenna, a heterodyne oscillator coupled to said detector for beating with the received waves to produce an intermediate frequency, an intermediate frequency amplier coupled to the output of said heterodyne deftector, a phase modulation detector and an amplitude modulation detector, couplings from the output of said intermediate frequency amplifier to the input electrodes of said detectors, and a phase rotation responsive device coupled to the outputs of said detectors, a contact arm mechanically linked to the armature of said phase rotation responsive device, a pair of -contacts oppositely disposed relative to said arm and adapted to alternately engage said arm depending upon the direction of torque produced `upon the armature of said device, an audio frequency oscillator coupled to said contacts and arranged to be con-l trolled therefrom, saidaudio oscillator generating one frequency when said arm engages one of said contacts and another frequency whensaid arm engages the other of said contacts, a circuit for modulating the amplitude of the heterodyne oscillator from .said audio oscillator, and headphones coupled tothe output of said amplitude detector.

6. In combination, a pair of sources of waves to be compared, means forbeating said waves together, an amplitude modulation detector and a phase modulation detector, circuits coupling the inputs of both of said detectors to the output of said means, and a two-phase rotation device vhaving one winding coupled to the output, of one detector and another winding coupled to the output of the other detector, a needle mechanically linked to the rotatable armature of said twophase rotation device, and a scale having index markings thereon over the length of which said needle moves.

'7. The method of comparing the relative frequencies of two alternating currents of different amplitudes and different frequencies, which comprises beating said currents together to obtainV two phase displaced output alternatingcurrent waves of reduced frequency, one of said two phase displaced waves being due to the amplitude modulations and the other phase displaced wave being due to the phase modulations of said .two alternating currents of different frequencies, and producing from said two phase displaced waves sound waves `whose frequency `is a function of both the difference in frequency between said two currents and the sense orfdirection ofthe frequency difference. y

8. In a'frequency comparison system, a source of reference frequency, a source of varying frequency, apparatus for comparing said frequencies comprising a frequency mixer coupled to both' of said sources, and means coupled to the output of q said mixer for assuring substantially constant relative magnitude of current derived therefrom, an amplitude modulation detector and a phase current, and a two-winding phase rotationrelay responsive to the phase relation of the -output currents from said two detectors, one winding of said relay being coupled to the output of said amplitude. modulation detector, while the other winding' of said relay is coupled to the output of said phase modulation detector.

-. 9. In a frequency comparison system, a. source of reference frequency, a source of varying frequency, apparatus for comparing said frequencies comprising a, frequency mixer coupled to both of said sources. an ampliner coupled to said mixer, and means for assuring substantially 'constant relative magnitude of current in the output of said amplier, an amplitude modulation detector coupled to the output of said amplifler, and a phase modulation -detector also coupled to the output of said ampliner, and a two-phase motory having a. pair orwindings coupled to the outputs of said two detectors.

. CLARENCE W. HANBELL REFERENCES CITED The following references are of record in the `tile of this patent:

UNITED sra'rns PATENTS' Number Name Date 1,934,400 Bollman .F.--'. Nov. 7, 1933 2,104,801 Hansel! Jan. 11, 1938 2,177,081 Gerhard Oct. 24, 1939 2,207,540 Hansell July 9. 1940 2,305,614 Goldstein Dec. 22,1942

' 2,351,548 Schwartzv June 13. 1944 2,377,326 Crosby June 5, 1945 

